Many communication systems, especially those connected with the switched telephone network, require a full duplex link. This has traditionally presented problems when a radio link is needed. Solutions either require frequency division of the two paths together with duplexing filters for "2 to 4 wire" splitting, or TDM which can only be easily implemented in systems which are already digital.
U.S. Pat. No. 4,238,850 issued Dec. 9, 1980 discloses a transmitter/receiver equipment for a single channel duplex communication system incuding a local oscillator means the frequency of which is the center frequency of the single channel signals, means for frequency modulating the local oscillator with first audio frequency signal, first and second means for directionally combining the local oscillator output in phase quadrature with audio frequency modulated signals received from the single channel whereby a major portion of the local oscillator output is transmitted from each combining means to the single channel and a minor portion of the local oscillator output together with the received signals is applied from each combining means to first and second mixing means.
This apparatus further includes, respectively, means for low pass filtering each of the mixed outputs of the first and second mixing means, means for amplifying each of the outputs of the low pass filtering means to a constant level, means for differentiating each of the outputs of the amplifying means, means for multiplying the output of each differentiating means with the input to the other differentiating means, means for subtracting one of the multiplier outputs from the other to provide the demodulated second audio frequency signal, means for subtracting a proportion of the first audio frequency signal from the demodulated signal and means for feeding the output of the local oscillator in combination with a phase quadrature output of the local oscillator to an aerial via which the signals frequency modulated with the second audio frequency are received.
In such equipment the receiver portion makes use of a so-called "zero IF" or "direct conversion" method of demodulation. Such a method is described in British patent specification No. 1,530,602--Vance--1. Briefly, local oscillator signals in phase quadrature at the carrier frequency are each separately mixed with the incoming audio modulated signal. The resulting signals have zero IF with the two sidebands folded over on each other at the baseband and extending in frequency from DC to the single sideband width of the original signal. The mixer outputs are low pass filtered and then amplified to a standard level. After amplification the two signals are separately differentiated. Each differentiated signal is then multiplied by the input to the other differentiator and one of the multiplier outputs is subtracted from the other.